Generally, there are several types of thicknesses of medium-soft (#2 or HB) black lead provided to use in known propelling pencils, for example, the black lead of which the diameter is 0.5 mm, 0.7 mm, and 0.9 mm. These black lead types need not be sharpened and can be used directly; but they have disadvantages that they are apt to break and are used in limited scope because of their single hardness type.
The hardness and strength of soft black lead types and colour lead types are so soft and weak, respectively, that they can not adapt to known propelling pencils because they are very apt to break with the same thickness as that of above medium-soft black leads. If these lead types are made into ones with thicker diameters there are also disadvantages in that they are not able to cooperate with the structure of the known propelling pencils and are inconvenient for a user to sharpen even if there is the adaptive structure of a propelling pencil.
And, future pencils shall not be wood holder pencils because they are not only inconvenient for a user to sharpen but also because they consume a large amount of quality wood.
A known pencil sharpener is not able to be employed to sharpen the leads of propelling pencils because the alignment between its blade and the lead is difficult to achieve.
In the structures of the known propelling pencils, a lead is kept only by the three claws at the front end of the lead holder, which restrict movement of the lead along its longitudinal axis. It, however, can rotate because the three claws hold it by only the three touching points. Thus, this is another reason why known pencil sharpeners are not able to adapt to sharpen the leads in known propelling pencils.